This disclosure relates to lasers, in general, and in particular, to high power, high brightness, monolithic fiber laser beam combiners that are suitable for laser weapons applications, including airborne tactical laser weapons systems.
Light, robust, high performance solid state laser weapons are desired for a broad range of military applications. None of the approaches currently being pursued, including slab lasers of various geometries and fiber lasers, is making progress rapidly enough to support near-term military applications.
One approach currently under development utilizes “slab lasers” of an appropriate solid material, such as Yttrium Aluminum Garnet (YAG), doped with an appropriate element which can support lasing. These dopants may include lanthanide elements, such as neodymium-(Nd), ytterbium-(Yb), erbium (Er) or thulium (Tm) or other elements. These “SSL” laser combiners utilize multiple slabs of doped YAG illuminated by laser diodes operating at an appropriate wavelength based on the adsorption characteristics of the doped slabs. These slabs are then located in a resonator cavity, in which a laser beam is generated. Depending on the configuration of the laser, multiple slabs may be used, and a combined laser beam produced thereby. The multiple beams are combined in such a way that the individual beams are in phase with each other. This causes them to act as if they were a single laser beam. The beam or beams produced must be of near-perfect beam quality so as to produce a maximum intensity on the target to which they will be propagated. The technology currently being pursued in slab lasers has been demonstrated beyond the 10 kW level and is expected to enable single or phased beams to be generated to the 100 kW level.
However, both Yb- and Nd-doped lasers operate at a wavelength close to 1 micron, a region in which the eye is extremely sensitive to retinal damage. This may limit their utility in areas in close proximity to people. While means exist for achieving operation in an “eye-safe” regime, these approaches typically produce lasers with much lower efficiencies and have not been demonstrated at high power.
Additionally, slab lasers have another drawback relating to their need for extensive cooling of the doped YAG slabs. Failure to cool adequately leads to rapid material failure due to the brittle nature of this material. Even before that point is reached, however, the material will thermally distort the beam to an unacceptable extent if the beam is to be propagated to a distant target. Such considerations therefore do not apply, for example, to SSLs used for welding applications, in which the beam is propagated over relatively small distances (e.g., a few inches or feet) but are important for laser weapon applications in which propagation distances are relatively long (e.g., miles.) The need for cooling makes it essential that the slabs used be very thin, which in turn, complicates both adsorption of the laser diode light and the extraction of the laser beam.
Thus, slab lasers are typically large and heavy compared to the kinds of platforms desired for laser weapons. This is in large part due to their low electrical-to-laser beam conversion efficiency, which requires that large numbers of laser diodes be used to pump the laser and that large cooling systems be provided to take away the heat. Supplying this power requires large power sources (e.g., batteries), and the thermal management capabilities required to support operations are also large and heavy, and accordingly, are not optimum for airborne weapons platforms, where weight is at a premium. Their low efficiency also limits the ability to scale the lasers because the cooling problems become more severe as the size and power of the hardware increase.
In another general approach, systems that combine the outputs of a number of “fiber lasers” have been developed for various applications, including laser welding. Fiber lasers continue to improve in power output, and provide another possible route to achieving light weight laser weapons. Because of their geometry, fiber lasers are typically capable of achieving much higher efficiencies of conversion of electricity into laser light. Further, their geometry typically enables better cooling than in slab lasers. Accordingly, a broad range of optical techniques are currently being pursued for fiber laser beam combining.
However, these approaches are also encountering some difficulties. Many of the more promising approaches from a technical standpoint require extraordinarily complex optical systems, which may not be acceptable for weapons applications. Additionally, fiber lasers have not yet been developed that have outputs beyond the kW-scale for beams with high brightness, although low-brightness fiber lasers have produced beams beyond the 10 kW-scale.
As those of skill in the art will appreciate, in addition to achieving high power in the combined laser beam, laser weapons require a combined beam that also has a high “brightness.” In this context, brightness refers to the power per unit area per unit solid angle subtended by the beam. Thus, to achieve a high brightness, the individual beams must be mutually coherent and combined in such a way as to produce a single-lobed far-field pattern with negligible side lobes.
Where high brightness is required, as it is for laser weapons, methods are needed to combine large numbers of fiber lasers into what are effectively single, high-brightness beams. Various techniques are currently being pursued in an effort to do so. One such approach utilizes a reference fiber laser as a standard, and then modulates each additional fiber laser that is slaved to the reference in a way that permits detection and correction of any phase errors. Each slave beam to be combined must be modulated at a distinct frequency and its phase error detected. This creates a system that is both electronically and optically complex. As with all of these approaches, achieving a combined beam of high brightness requires that each individual fiber laser beam has excellent beam quality. This imposes limitations on the optical configuration and penalizes overall system efficiency.
In a related approach, the multiple fiber laser beams are optically phase-matched through electronic feedback means that enable continuous adjustment of phase. This approach becomes extremely complex for phasing of a large number of beams. As in virtually all approaches being pursued, this technique relies on the fiber laser beams being individually of high brightness. This requirement generally limits the efficiency of the fiber lasers. In addition to that effect, there are also significant losses in efficiency that are encountered due to imperfections in the phasing of the beams to each other.
In another approach, multiple fiber laser beams having slightly different wavelengths can be combined by the use of a diffraction grating. The alignment of these beams into a single combined beam must be carefully controlled such that the beams do not effectively separate over the long propagation distance to the target characteristic of laser weapons. This typically requires coalignment to ˜1 microradian, which is a very difficult standard to achieve in practice.
Yet another approach to fiber laser beam combining relies on a property of waveguides to achieve phasing of the beams. However, as will be appreciated, where 100 or more individual fiber laser beams must be combined, the mechanical tolerances are extremely tight and the complexity can become impractically high.
As a practical matter, fiber lasers generally are not currently scalable beyond the 1-10 kW range. Typically, high brightness can be achieved only at the low end of this range, while “welding lasers” can be scaled more effectively to the higher end. Typically, the geometry of fiber lasers can produce higher efficiency than slab lasers, because the long fiber can more efficiently adsorb the laser diode pump light. However, this higher efficiency is significantly degraded when “single mode” operation is required to provide a high-brightness source that can be optically combined into a high-power, high-brightness laser.
Beam combining is required to achieve laser weapon power levels. This requires the optical combining of dozens or perhaps hundreds of individual fibers into a single high-brightness projected beam. This increases the optical complexity of the system substantially, and is potentially a major source of reduced efficiency and beam brightness. Thus, the technology needed to combine multiple fiber lasers into a single, high-brightness beam remains elusive, with none of the approaches currently being pursued assured of achieving the performance and robustness needed for practical laser weapons. The approaches being pursued all have limitations in terms of being able to phase the beams and of achieving a high brightness in the combined output beam.
Thus, there is a long-felt but as yet unsatisfied need for a technology that is practical for the implementation of a high power, high brightness laser weapon system that is light in weight and compact enough to be carried aboard an aircraft.